C57BL/6 mice infected with the neurotropic rJ2.2 variant of the JHM strain of mouse hepatitis virus (rJ2.2) develop acute and chronic demyelinating encephalomyelitis and thereby serve as a useful model for the human disease multiple sclerosis. The overall objective of this project has been to understand virus-host interactions, to enhance virus clearance and minimize tissue damage. In the past funding period, we concentrated on aspects of the T cell response that were pathogenic but protective and others that were regulatory, dampening CD4 T cell-mediated immunopathological changes. However, before a T cell response can clear virus, the host must mount a well-orchestrated and robust innate immune response. Our recent work has focused on two aspects of this early innate response, both involving microglia, which are the resident myeloid cells in the CNS. First, we showed that microglia are critical for protection. Although microglia have important roles in immune surveillance and maintenance of homeostasis in the CNS, their role in the context of infection remains ill-defined. Our results show that they are key for protection from rJ2.2 infection. Second, we found that signaling of a single eicosanoid, prostaglandin D2 (PGD2), through its DP1 receptor, found on myeloid cells including microglia, was required for optimal survival in rJ2.2-infected mice. In the absence of DP1 signaling, type 1 IFN (IFN-I) expression was delayed and IL-1? expression was increased, suggesting that PGD2/DP1 signaling was critical for a balanced immune response. The absence of PGD2/DP1 signaling resulting in dysregulated microglia function. Therefore, the central objective of this project is to better understand the many roles that microglia have in enhancing the host response to viral infections in the brain and thereby improving outcomes in the context of viral encephalitis (and ultimately, demyelinating disease). This objection will be addressed in the following specific aims. Specific Aim 1: To determine the basis for lethal outcomes in microglia-depleted rJ2.2-infected mice. In this aim, mice will be treated with PLX6522, which depletes microglia. We will monitor virus clearance and immune function after treatment. We will assess whether macrophage numbers, function or gene expression is changed in the absence of microglia. Specific Aim 2: To determine the role of PGD2/DP1 signaling in microglia in rJ2.2-infected mice. In this aim, we will assess effects of PGD2/DP1 on viral control and the host immune response. We found that a PYRIN domain only protein, PYDC3, was downregulated in microglia of mice lacking DP1 signaling. PYDC3 has similarities to IFN-I-inducible human proteins that regulate inflammasome formation and activation but homologous proteins have not been identified in mice. One of the goals of this aim is to understand the interactions and roles of prostaglandin D2 signaling, IFN-I expression and inflammasome function.